Method for the production of a biocompatible polymer-ceramic composite material with a predetermined porosity

ABSTRACT

Method for the production of a biocompatible polymer-ceramic composite material with a predetermined porosity, designed and determined a priori, which includes a first phase (a) of the production of a suspension of a bioceramic material in distilled water, a second phase (b) in the which a compact of the bioceramic material containing a desired quantity of water is obtained from the suspension, and a third phase (c) in the which the compact is mixed with a polymeric material and/or a liquid monomer.

TECHNICAL FIELD

The present invention relates to a method used to obtain a biocompatiblepolymer-ceramic composite material of a predetermined porosity, designedand determined a priori.

BACKGROUND ART

It has been known for some time that polymethyl methacrylate(PMMA)/calcium phosphate type porous composites may be employed in aseries of applications, such as the filling of bone voids or as drugdelivery systems for the controlled release of pharmaceuticals. In fact,these composites display a proven biocompatibility, and at the same timethey succeed in wedding the mechanical resistance characteristicsinherent in the polymeric materials such as PMMA with thebio-reabsorption characteristics of bioceramic materials such as calciumphosphate.

A determining aspect of such polymer-ceramic composite materials isporosity, which can be a deciding factor both for the mechanicalcharacteristics and for the functional characteristics of the compositematerials themselves. In fact, porosity allows the composite material tohost staminal cells, proteins that stimulate the colonisation of thepatient's staminal cells, antibiotics, growth elements, and otherbioactive substances that in general promote the processes ofattachment, osteointegration and/or reabsorption of the compositematerial.

Further, designing the porosity is particularly important, since thepores must assume specific characteristics both in shape and size as afunction of the various applications of the material. In fact, the roleof porosity and the degree of interconnection between the pores has beenrecognised as an important parameter both for the reconstruction of bonetissue inside the implanted polymer matrix and for the release periodsof any pharmaceuticals inserted in the composite material.

Generally, biopolymeric porous materials are created using foamingagents or by inserting in the polymer matrix powders of particles thatcan be dissolved at a later stage, as, for example, soluble salts orgelatin microspheres.

The solid particles destined to create the porosity can be introduced inthe melted polymer, in the monomer or mixed with the solid prepolymerbefore the polymerisation or reticulation reaction. During this phase,difficulties may arise due to the possibility that a few particles canremain isolated and therefore do not contribute to the formation ofporosity, or that the area of contact between two particles can be verysmall. In such cases, the periods for the removal of the solid increase,the diffusion of bodily fluids is inhibited and a large fraction ofporosity can therefore prove useless from the point of view of cellularcolonisation. Porosity created using foaming agents can also entail thesame type of difficulty, with the formation of a large fraction of cellsthat are closed or only virtually connected through fractures in thesurfaces that connect one cell to another.

With the aim of resolving these difficulties, the use of biocompatibleand bioabsorbable liquids has been proposed. In particular, anespecially effective method according to U.S. Pat. No. 4,373,217 is theadvance treatment of the ceramic material powders with these liquids,aiming to fill the porosity, at least in part, in order to avoid itbecoming filled with monomer during the initial phases ofpolymerisation, consequently impeding the subsequent dissolution of theceramic material and therefore the creation of the desired porosity inthe final composite. Further, the article “Use of α-tricalcium phosphate(TPC) . . . ” by D. T. Beruto, R, Botter in the Journal of BiomedicalMaterials Research 49, 498-505, 2000, discloses the use of distilledwater to create aqueous dispersions of the bioceramic material utilised,which are subsequently mixed with the polymeric material and with liquidmonomer. The use of these dispersions, beyond avoiding the difficultiesexplained above and guaranteeing the generation of good porosity, alsoprevents the bioceramic materials used, as for example calciumphosphate, from absorbing part of the liquid monomer and removing itfrom polymerisation with the successive risk that it be released itselfin the patient's circulatory system. The liquids utilised, in fact,being miscible with the bioceramic material and non-miscible with themonomer or with the polymer used, impede the contact of the latter withthe bioceramic material itself.

The techniques utilised up to now, which call for the creation ofaqueous dispersions of the bioceramic material, notwithstanding the factthat they succeed in resolving the difficulties described above, arenonetheless incapable of allowing for the design and achievement of afinal porosity of the predetermined composite.

DISCLOSURE OF INVENTION

The aim of the present invention is to realise a method for theproduction of a polymer-ceramic composite material using which it willbe possible to predict and design the porosity of the final compositematerial.

According to the invention therefore, a method is created to obtain abiocompatible polymer-ceramic composite material of a predeterminedporosity, said method comprising a first phase (a) of the production ofa suspension of bioceramic material in distilled water, and ischaracterised by the fact that it also comprises a second phase (b) inwhich a compact of said bioceramic material containing a desiredquantity of water is obtained from the suspension; said compact is thenmixed in a third phase (c) with a polymeric material and/or with aliquid monomer.

Preferably, the desired quantity of water is calculated on the basis ofa combination of a calibration curve of the water contained in a compactof bioceramic material as a function of the different level ofcompaction used to create the compact, and from a calibration curve ofthe porosity of a polymer-ceramic composite as a function of thequantity of water contained in the compact used in creating thepolymer-ceramic composite itself.

Preferably, the compact is obtained using a sedimentation in centrifugeoperation.

Preferably, the polymeric material utilised is polymethyl methacrylate,the liquid monomer is methyl methacrylate and a suspension of aprepolymer in the monomer is prepared in advance, which is then mixedwith the compact containing the predetermined quantity of water.

Preferably, the bioceramic material is constituted of calcium-deficienthydroxyapatite or tricalcic-phosphate α.

Preferably the bioceramic material should be used with a definedgranulometry. For instance, diameters between 1 μm and 200 μm can beused.

More Preferably the diameter range for the selected powder can becomprised between 1 μm and 10 μm or 10 μm and 50 μm or 50 μm and 100 μm.

According to a preferred embodiment of the invention, the preparation ofthe tricalcic-phosphate α comprises a final rapid cooling phase and asieving phase, possibly after grinding, in order to collect particles ofirregular shape, approximately ranging between 1 μm and 10 μm in size.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics of the invention will be apparent from thefollowing description of a few examples, provided for illustrativepurposes only and that are not limitative, and which will be describedwith reference to the attached figures, among others, in which:

FIG. 1 is a graphic that shows the trend of the total volume WR of waterretained by the compact at the end of the sedimentation trials as afunction of the centrifugal acceleration used;

FIG. 2 is a graphic that represents the trend of the volume WB of bondedwater retained by the compact at the end of the sedimentation trials, asa function of the centrifugal acceleration used; and

FIG. 3 is a graphic which represents the trend of the porosity of thepolymer-ceramic composite as a function of the water retained by thecompact used in the production of the composite itself.

FIGS. 4 and 5 are comparative graphics representing the quantitativerelease of an antibiotic from composite PMMA/α-TPC sedimented andcentrifuged;

FIG. 6 shows the imbibition rate of water by Wicking Technique compositePMMA/α-TPC;

FIGS. 7 and 8 shows the pores diameters and volumes for compositePMMA/α-TPC obtained according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES

Two series of composites, A and B, were prepared, which aredistinguished from each other by the use of two different bioceramicmaterials with the aim of highlighting how the design of the porosityalso depends on the type of bioceramic material utilised. Specifically,the series of composites indicated by the letter A calls for the use oftricalcic-phosphate α (α-TCP), while the series of composites indicatedwith a B calls for the use of calcium-deficient hydroxyapatite (CDHA).

In particular, in the examples shown below, the design and prediction ofthe porosity of the biocompatible polymer-ceramic composite material wasobtained using a method including the following phases:

-   -   (a′) producing a suspension of a bioceramic material, with a        selected granulometry, in distilled water;    -   (b′) starting from identical volumetric quantities of initial        suspension, obtaining from the suspension a series of compacts        of the bioceramic material containing different quantities of        water;    -   (c′) mixing each of the compacts thus obtained with an identical        quantity of a polymeric material and/or a liquid monomer in        order to obtain a porous geometric solid of pre-defined        dimensions;    -   (d′) for each compact, calculating the porosity of the solid        obtained from it; and    -   (e′) correlating the porosity with the residual water content of        the compact.

Example 1

Series of Composites A

Example 1a

Preparation of the Compacts

An inorganic tricalcic-phosphate salt α (α-TCP) was prepared using areaction in the solid state between anhydrous CaCO3 and CaHPO4. Aftermixing, the dibasic calcium phosphate powders were heated in a mufflekiln to 1573 K and at the end of the reaction were rapidly cooled inorder to stabilize the α phase. After cooling, the powder was sievedusing a 60 mesh sieve, and the fraction of powder that passed throughthe sieve was analyzed with X-ray diffraction, confirming the αstructure of the powder. The average size of the grains wasapproximately 10 micron.

The inorganic salt α-TCP was dispersed in the aqueous phase with a solidphase volumetric concentration equal to 10%. From the dispersions thusobtained, a total volume equal to 12.7 cm³ was extracted. This volumewas treated in a centrifuge and subjected for a pre-defined period of 15minutes to a value of acceleration (Xg). The same procedure was repeatedvarious times, subjecting the various dispersions obtained to variousvalues of acceleration (Xg). At the end of each centrifugation period, a“compact” and an aqueous phase were obtained. For each compact obtained,the aqueous phase was separated from the compact and the residual watercontent of the compact was determined by weight. FIG. 1 shows the watercontent WR (expressed in cm³/grams of dry powder) remaining in thevarious compacts obtained for different values of Xg.

The water content WR is formed by water still relatively free betweenthe cracks in the grains and by water bonded by capillary andsuperficial forces to the inorganic matrix. According to this invention,the major datum in predicting the final porosity of the composite isnonetheless not total water WR, but water WB that is bonded by forces ofvarious natures to the ceramic matrix. This quantity is defined by:WB=WR×p 1  (1)

-   -   where, for every Xg, WR is the total water inside the compact,        WB is the bonded water and p1 is the probability that the water        is bonded. This probability is complementary to the probability        of finding free water. The fraction of free water inside each        compact therefore represents that part of the water that is        susceptible to leak from the ceramic matrix under mild force.        When a specific compact, obtained by treating the dispersion to        an Xg acceleration, is subjected to a further force of dXg, the        first water to exit the ceramic matrix will be the least bonded        portion. An index of this quantity is given by the value of the        derivative of the curve in FIG. 3 calculated for each        experimental abscissa Xg. Therefore, a reasonable formula to use        in calculating WB is:        WB=WR×[1−k(dWB/dXg)]  (2)    -   where k is a parameter chosen as a function of the dispersion of        the experimental data in order to optimize the linearity of the        relation.

FIG. 2 illustrates the results of the calculations performed as above onthe basis of the experimental results of FIG. 1, in order to evaluatethe bonded water content WB corresponding to each experimental contentWR.

Example 1b

Preparation of the Composites

The composites (PMMA/phosphate) were produced using pre-polymerized PMMAand monomer (MMA) powders currently on the market-such as the type usedas orthopaedic cement-utilising a well-known methodology, which issummarized below.

1.33 g of monomer (MMA) were placed in a glass beaker, and to this 4 gof PMMA were added in a single solution. After approximately 10 secondsof shaking, the mixture achieved a soft, runny and homogenousconsistency. A compact prepared in example 1a was added to thesuspension. The resulting composite paste was rendered homogeneous byrepeatedly folding the contents of the beaker back into itself forapproximately 40 seconds. At the end of this operation, the content wasextracted and formed between two flat plates of glass to a thickness ofapproximately 4 mm. After an hour at room temperature, the hardenedcomposite was dried in an oven at 60° C. for eight hours, andsubsequently was cut into regular parallelepiped shapes. Using the sameprocedure, various composites obtained from the different compactsprepared in example la were produced, as shown in Table 1, which alsoshows the amounts of additives (known) utilised to optimize thepolymerisation reaction.

The total volume of each of the composite products was measured usinghelium pycnometry after drying in a vacuum at room temperature. Theinternal porosity (P) was determined from the difference between theapparent volume of the trial (Va) determined geometrically and the realvolume (Vr) determined with the pycnometer.P=Va−Vr

FIG. 3 shows the porosity (P) of the composites as a function of thewater content (expressed in cm³/grams of powder) remaining in thevarious compacts from which the composites themselves were obtained.

Example 2

Series of Composites B

Example 2a

Preparation of the Compact

The procedure described in example 1a was repeated with the differencethat the inorganic salt used, rather than α-TCP, was calcium-deficienthydroxyapatite (CDHA).

As in example 1a, FIG. 1 shows the water content (expressed in cm³/gramsof dry powder) remaining in the various compacts obtained at differentvalues of Xg, and FIG. 2 contains the corresponding values of WBcalculated as in example 1a.

Example 2b

Preparation of the Composite

The procedure described in example 1b was repeated. However, thecompacts prepared in example 1b were used. The exact amounts used interms of weight are shown in Table 1.

As for example 1b, FIG. 3 shows the porosity (P) of the composites as afunction of the water content (expressed in cm³/grams of powder)remaining in the various compacts from which the composites themselveswere obtained.

TABLE 1 Component PMMA MMA 99.1% + N—N CDHA, 97% + 3% dimethyl-p- αTCPbenzoyl toluidine 0.9% + Dry Residual peroxide Hydroquinone 75 ppmpowders water Quantity 4 1.33 1.8 From 1.4 to 2

Example 3

Methodology for Predicting Porosity

A very simple procedure for obtaining a desired porosity of a compositeresults from the examples given above. Once the desired porosity and thebioceramic material to be used have been established, using an“adjustment” graphic like the one illustrated in FIG. 3 calculated inadvance for the appropriate bioceramic material, we look for the amountof water WB that the compact constituted of the bioceramic material mustcontain. Once the quantity of bonded water that must be contained in thecompact has been established, we find the centrifugal acceleration usedin preparing the compact by using a second corresponding adjustmentgraphic, like that illustrated in FIG. 2.

In the end it is clear that, in the event that another method ofcompaction is used (for example press filtering, grinding, etc.), theparameter to be considered will not be the centrifugal acceleration buta parameter inherent to the method selected.

Example 4

Methodology for Choosing an Appropriate Bioceramic Composite to Obtain aCertain Porosity

In order to choose the most suitable type of commercial powder forproducing a composite with PMMA of a desired porosity, we will proceed,on the basis of the previous examples, as follows:

-   -   Phase 1. Perform the calibration, in the centrifuge or with a        similar technique, of the aqueous dispersions of the commercial        powders under analysis;    -   Phase 2. Construct the graphic WB vs. Xb or other variable,        according to the technique used for compaction;    -   Phase 3. From among the initial powders, choose that which has a        water content equal to WB. If it does not exist, prepare a        compact, beginning with any of the powders, subjecting the        initial dispersion to the corresponding acceleration Xg        according to the adjustment curve;    -   Phase 4. Prepare the mixture of the compact containing the        desired quantity of bonded water, the pre-polymerized PMMA        powders and the monomer according to the examples 2a and 2b.

Example 5

Preparation with Prepolymer Dispersions

Examples 2a and 2b are repeated using a variation of the methoddescribed, consisting in pre-mixing the PMMA prepolymer powder with themonomer to obtain a concentrated polydispersed suspension of sphericalPMMA particles with an average diameter of between 15 and 40 microns andan average molecular weight of between 250,000 and 350,000 uma in ahydrophobic liquid consisting predominantly of MMA monomer. Further, weuse compacts obtained by starting with bioceramic component powders withaverage granulometry of 10 microns that are obtained by grinding initialpowders with higher granulometry, between 30 and 45 microns.

Example 6

Antibiotic Release from Preparation with Prepolymer Dispersions

2 Mixtures with the same procedure described in example 5 have beenprepared. α-TCP is added in different amounts (28% and 31% w/w powdercomponent).

Either sedimentation or centrifugation is applied.

4 different types of specimen are obtained:

28% (α-TCP Sedimented, 28% α-TCP centrifuged; 31% α-TCP Sedimented, 31%α-TCP centrifuged.

The specimen are dried for two hours at 90° C. Once dried the specimensare weighted and then immersed in an antibiotic solution (2.5% w/wgentamicin/water) for 30 mins.

The specimen are newly weighted to measure the amount of solutionloaded.

Each specimen is placed in a different container with a known amount ofsterile saline solution.

Takings of the saline solution are made at definite times. After eachtaking the saline solution is refreshed with new one.

The takings are then checked for antibiotic release using the Agar-welldiffusion method.

The results show clearly that centrifugation permits to control thekinetics of release (FIG. 4); the amount of α-TCP instead influences theabsolute value of antibiotic solution release (FIG. 5).

Example 7

Qualitative Control of Pore Dimensions

Mixtures with the same procedure described in example 6 have beenprepared.

4 different types of specimen are obtained.

28% α-TCP Sedimented, 28% α-TCP centrifuged; 31% α-TCP Sedimented, 31%α-TCP centrifuged.

The specimen are dried for two hours at 90° C. Once dried the specimensare weighted and then immersed in mercury for porosimetry determination.

The results show that the dimensions of pores are for every mixturescomprised between 2 μm and 10 μm, with a maximum ranging between 3 μmand 5 μm. The granulometry of α-TCP (average 10 μm) influences thedimension of the pores in the matrix FIG. 7.

Example 8

Control of the Imbibition Properties for Preparation with PrepolymerDispersions

Mixtures with the same procedure described in example 6 have beenprepared.

4 different types of specimen are obtained.

28% α-TCP Sedimented, 28% α-TCP centrifuged; 31% α-TCP Sedimented, 31%α-TCP centrifuged.

The specimen are dried for two hours at 90° C. Once dried the specimensare weighted and then partially immersed in distilled water for dynamicweight determination. The “Wicking technique” is applied.

The results presented in FIG. 6 show that the amount of α-TCP affectsthe absolute value of the water absorbed by the specimen. Thecentrifugation affects the speed of absorption.

Example 9

Quantitative Control of Porosity

Mixtures with the same procedure described in example 6 have beenprepared.

4 different types of specimen are obtained.

28% α-TCP Sedimented, 28% α-TCP centrifuged; 31% α-TCP Sedimented, 31%α-TCP centrifuged.

The specimen are dried for two hours at 90° C. Once dried the specimenare weighted and then immersed in mercury for porosimetry determination.

The results presented in FIG. 8 show that the volume of mercury forcedin the material is directly dependent on the α-TCP content and inverselydependent on centrifugation.

The results achieved are similar to the previous results, but thecomposites obtained also display a better interconnection in theporosity achieved, as evidenced by comparative experimentation with thecomposites obtained in examples 2a and 2b, performed using the “Wicking”methodology (Z. Li et al. “Wicking technique for determination of poresize in ceramic material”, J.Am.Ceram. Soc. 77, 2220-22(1999)).

The examples described thus illustrate that the composite materialsobtained using the methodology of this invention are especiallywell-suited both for the production of temporary prostheses withcontrolled release of pharmaceuticals, which can be achieved withpredetermined kinetics thanks to the possibility of determining theproduct's porosity in advance, as well as for highly osteo-conductivebone substitutes.

Further, it is evident that using these materials, other types ofapplicative products can also be produced, in all cases that require arigorous control of porosity, as, for example, with semi-permeablemembranes. Finally, it is also clear that the methodology is applicableto any type of porous bioceramic material.

1. Method for predicting and designing the porosity of a biocompatiblepolymer-ceramic composite material, characterized by the fact that itincludes the following phases: (a′) producing a suspension of abioceramic material in distilled water; (b′) starting from identicalvolumetric quantities of initial suspension, obtaining from thesuspension a series of compacts of the selected bioceramic materialcontaining different quantities of water; (c′) mixing each of saidcompacts obtained with an identical quantity of a polymeric materialand/or a liquid monomer in order to obtain a porous geometric solid ofpre-defined dimensions; (d′) for each compact, calculating the porosityof the solid by obtained from it; and (e′) correlating said porositywith the compact's residual content in water.
 2. Method for predictingand designing the porosity of a biocompatible polymer-ceramic compositematerial according to claim 1, characterized by the fact that the phaseof obtaining said compacts, which have different residual watercontents, from said suspension, is performed by centrifuging saidpre-determined volumetric quantities of said suspensions at various,progressively increasing, accelerations.